It
had been a
night of live
blues at a
little club
downtown.
Afterward some
of us had gone
back to my
place and lit
up the system
with some
vintage
Chicago blues.
Suddenly, Pete
asks,
"Can you
turn it
up?"
Unfortunately,
I had to admit
that we were
just about at
the dynamic
limits of the
superb little
Pass Labs
Aleph 3.
Though rated
at only 30
watts per
channel, when
paired with my
Von Schweikert
Research VR 4
Generation II’s,
with their
relatively
high 91 dB/W
efficiency,
they have no
problem
filling my
15.5’ × 23’
room –
within reason.
With the
impetus of
that seemingly
innocent
question I
knew it was
time to
meddle! After
a few quick
e-mails to
Albert Von
Schweikert,
designer of my
speakers, and
HAL, my
engineering
audiophile
buddy, I was
ready to rock!

Last
spring, HAL
had alerted me
to the deep
discounts
being offered
on the Hafler
Trans-Nova
series of
amplifiers by HCM
Audio
Online. Why
were these
Hafler amps so
attractive for
my project?
Let me count
the ways. They
have excellent
overall sonic
performance
for their
price,
especially in
the low
frequencies,
which was an
important
consideration.
They had their
own
front-panel-mounted
volume
controls, also
critical for
our project.
Finally, they
are quite
affordable,
especially the
B-stock units.

What
is a B-stock
amplifier, you
ask? Simple.
Typically, it
is a unit that
failed in the
field, was
replaced under
warranty and
came back to
the shop for
repair. Once
fully repaired
to factory
specification,
because they
frequently
show some
small signs of
use, they
cannot be
resold as new.
As B-stock
units, they
are
electronically
perfect and
carry a full
warranty. All
the
performance at
something like
50% of the
retail price—what’s
not to like?
The B-stock 75
watt-per-channel
P1500
Trans-Nova I
selected,
which retails
for $599, was
just $300.

Now,
you certainly
don’t have
to buy a new
amp if you
happen to have
a suitable one
lying around.
When using two
identical
amps, you
would likely
opt for what
is known as a
Vertical Biamp
configuration.
In this
application,
aside from
maximizing
channel-to-channel
separation,
you would also
negate any
problems with
input to
output
impedances,
level
matching, and
so on.

Here
we will be
addressing the
more common
biamplification
configuration,
using one amp
for bass
reproduction
and yet
another for
the high
frequencies.
In this
configuration,
you will want
to use at
least one
amplifier with
output
controls,
preferably the
low frequency
amp.
Independent
volume control
for at least
one amp is
crucial,
because
without it you
will have no
chance of
matching the
outputs to
balance the
performance.
It will also
be important
for the amp
used for bass
reproduction
to have more
gain than the
one you will
be using for
the higher
frequencies.
If you’ve
got a suitable
amp lurking
about, or know
an audio bud
that has some
extra power
packed away in
storage, you
are ready to
move.

What’s
What, and Why

Let’s
take a look at
the spectrum
of human
hearing. It is
accepted that
we are capable
of hearing
from 20 Hz to
20 kHz. If we
start at 20 Hz
and break this
audible
spectrum into
musical
octaves, we
get the
following
results.

It
is significant
to note that
EACH octave
requires the
SAME
acoustical
power as any
other. This
means that it
takes the same
percentage of
the amplifier’s
available
power to play
the 10,240
cycles between
10,241 Hz and
20,480 Hz (the
top octave),
as it does the
20 cycles from
20 Hz to 40 Hz
(the bottom
octave).
Mathematically,
this is
expressed as a
logarithmic
relationship.
What does all
this mean? Let’s
say we are
listening to a
symphonic work
and we are
pleasantly
cruising along
to the
delicate sound
of a triangle
using 5 watts
of available
amplifier
power.

Now
a bass drum
kick hits. You
suddenly need
ten times that
5 watts of
coasting
power, or 50
watts, to
recreate that
bass drum
whack at the
same volume
that you hear
the struck
triangle! If
we are using a
7-watt SET
Triode, or
other
similarly
low-power
amplifier
being played
at a realistic
volume, it is
easy to see
that the
sudden demand
for power to
recreate this
bass note will
distort the
output signal
significantly.
This resultant
form of
distortion
here is known
as clipping,
because it
"clips"
off the top
and bottom of
the waveform.
If, on the
other hand, we
are using a
100-watt
amplifier, we
still have
power to
spare!

Although
you can
biamplify
without the
use of a
crossover
filter like
the one under
discussion
here, you will
gain several
significant
advantages by
doing so.
Without such a
filter in
place, both
amplifiers are
reproducing
the full
spectrum of
frequencies,
20 Hz to 20
kHz. By
relying ONLY
on the speaker’s
internal
crossovers to
filter
unwanted
information
away from the
respective
drivers, we
are simply
expending the
unused power
through the
crossover.
Though this
works just
fine, it is a
tremendous
waste of
amplifier
power!

Let’s
take a look at
a graphic
example. If
the speaker
system we wish
to biamplify
will be
crossed over
at a 640 Hz
(not too
likely, but
selected for
its clarity in
this example),
then without
the use of
such a filter,
each amplifier
will be
generating the
entire audible
frequency
spectrum. Now,
because of the
way the
drivers in our
model use this
power, with
only one half
of the
spectrum
needed for the
low
frequencies
and the other
half necessary
for the highs,
each amplifier
would be WASTINGfully one
half of its
acoustic power.
What if we
were to ask
the amplifiers
in question to
ONLY amplify
the portion of
the signal
their
respective
driver
complement
will be
reproducing?
See where we
are headed?

If
we employ a
high-quality
crossover
filter before
the two
amplifiers,
with the
intent being
the relief of
the physical
duty of
reproducing any
frequencies
other than
their
respective
driver
compliments
will be
responsible
for, we
achieve two
very
substantial
results. We
have
considerably
more effective
use of the
absolute power
available to
us and we have
significantly
reduced the
demand on
each, thereby
enhancing
their
respective
sonic
capabilities.

Let’s
take a more
realistic
model like my
own speakers
that cross
over at 120 Hz
between the
woofer section
and the
mid/tweeter
module. If,
through
implementing a
high-pass
filter, we
eliminate the
need for the
high-frequency
amplifier to
reproduce the
bottom 2½
musical
octaves (from
20 Hz up to
120 Hz), we
have freed it
of 25% of its
most demanding
workload!
Since
low-frequency
reproduction
places the
most severe
load on an
amplifier,
extracting the
most current,
it puts a
substantial
drain on the
power supply.
Relieving the
high-frequency
amplifier of
this stressful
duty results
in wonderful
sonic
benefits, such
as greater
ease, detail,
and clarity.
Win/Win, eh?

Now
let’s take a
look at the
other side of
the coin. What
advantages are
gained by
reducing the
duties of the
low-frequency
amp with the
application of
the 120 Hz low-pass
filter.
Our low-pass
filter
prevents the
top seven and
a half octaves
(121 Hz to
20,480 Hz)
from ever
reaching the
low-frequency
amp; that is 75%
of the
previous
full-frequency
load! We
can now devote
all
this newfound
power to the
accurate and
powerful
recreation of
ONLY the
bottom 2½
octaves. I
hate to repeat
myself, but
clearly a
Win/Win
circumstance
once again.

Number
Crunching

Between
Albert and
HAL, I got all
the science
and assistance
I needed. Our
filter of
choice is a
shallow-slope
single-pole 6
dB per octave
type of filter
for both high-
and low-pass
branches. By
definition we
have a
Butterworth or
Linkwitz-Reilly
alignment
filter in this
case. Why
single-pole?
Well, most
would agree
that it is the
least invasive
on the signal,
creating the
lowest level
of phase shift
and
degradation on
the low-level
signal
coursing
through it.
The equation
for finding
the -3 dB
point for our
crossover is:

F-3
dB = 1 / (2
x PI x R x
C)

Where:F-3 dB is
the -3 dB
point of the
circuit filter
point in Hz.
R is the total
series or
parallel
resistance
used in ohms.
C is the
capacitance in
Farads.

Notice
that in the
Resistance
branch of the
equation that
I said total
series or
parallel
resistance. If
the preamp has
a relatively
high output
impedance
(>100 ohms)
and the series
resistance is
low (<1000
ohms) for the
high-pass
filter branch,
the -3 dB
point will be
off the
calculated
value. You can
correct for
this by including
the output
impedance of
your preamp
(check your
owners
manual).
Similarly, for
the high-pass
function, you
can just use
the input
impedance of
the amp,
measured in
ohms, for R.

It
is a little
more difficult
to calculate
the low-pass
value since
you have both
attenuation
and filter
roll off to
work with. You
can add the
series R and
then parallel
the input
impedance with
the C. This is
essentially a
shelf filter
since it drops
the entire
band of
operation.
Keep in mind
that output
impedance
comes into
play as level
offset in the
low-pass
filter. If you
add a series
resistor in
the low-pass
section, then
the impedance
is the sum of
the series
resistor and
the input
impedance of
the amp. It is
not totally
resistive, but
close enough
for our
purposes.

This
is one reason
why you will
want to have
more gain in
the bass amp
than the
treble amp.
Matching
levels will be
very tough to
do unless you
actively
buffer the
low-pass
filter output
before the amp
input (way
beyond the
scope of this
do-it-yourself
[DIY]
article), or
have level
controls in
one or both
amps. See—I
said it was
crucial to our
project.

My
chosen flex
point was 120
Hz, because
that is
crossover
point between
the bass and
midrange/high-frequency
modules in my
Von Schweikert
VR 4 Gen II’s.
This value,
computed for
my equipment,
yields
approximately
0.47 micro
Farads for the
capacitance
and about 2.2
k-ohms for the
resistance.

Okay,
so now that we
have the
theory out of
the way, let’s
have some fun
and get down
to building
stuff, shall
we? We want to
build the
following
circuit.

Getting
the Goods

I
spent just
over 1½ hours
assembling the
boxes and
another half
hour doing the
final
balancing. If
you are not a
seasoned
solder sniffer,
you may need 4
to 5 hours for
all of this,
so slate your
time
accordingly if
you have other
obligations,
okay?

For
this project
you will need
a good
soldering
iron, a drill,
several drill
bits (one
¼" and
one smaller
pilot bit),
diagonal
cutters, a set
of needle nose
pliers, some
good solder,
and about a
foot or so of
high quality
hook-up wire.
I used some
Harmonic
Technology
Single CrystalTM
22-gauge
silver wire
that I had on
hand. You can
opt for any
high-quality
type of wire.
You will also
need a
Volt/Ohm meter
and a Radio
Shack (or
similar)
Decibel meter.
Beside a
second amp,
you will also
need two more
sets of
RCA-type
interconnect
cables. Use
the best ones
you can
afford. Our
raw components
will be
acquired from
two places,
Radio Shack
and The Parts
Express—the
hobbyist
friendly
division of
Sonic
Frontiers.

At
the Shack, you
will want to
pick up two
black plastic
experimenter’s
boxes and
three sets of
gold-plated,
chassis-mount
phono jacks.
The
"Project
Enclosures"
I used, part
number
270-1801, are
a scant
3" x
2" x
1". Given
the size of
the capacitors
I chose, this
makes for a
pretty tight
fit. If you
are NOT
an experienced
DIY’er, opt
for a larger
pair of
experiment
boxes. Trust
me, the small
boxes force a
snug but cozy
fit that might
frustrate all
but the most
experienced
and patient
solder hounds.
If you opt for
the larger
boxes, plan to
double the
amount of
hook-up wire
you will use,
as you will
likely need to
extend the
capacitor and
resistor
leads, as well
as make the
ground-leg
connections.

You
may wonder why
I am using the
inexpensive
Radio Shack
jacks rather
than some of
the more
expensive
"audiophile-approved"
versions that
are available.
Quite simply,
I vastly
prefer the
lowest mass,
least complex
plating scheme
I can find.
These fill the
bill perfectly
and have been
my choice for
such projects
for some time.
Feel free to
use larger,
sturdier, and
more expensive
jacks. If you
do, don’t
use the small
project
enclosure; you
won’t have a
chance of
cramming
everything in.
Oh yes, and
don’t blame
me for the
excessive
signal
attenuation.

For
the actual
electronic
components,
point your
browser to The
Parts
Connection
and look at
the online
catalog with
the Adobe
Acrobat Viewer
(available
free at www.adobe.com).
Make your
parts
selection, or,
if you are
using the same
crossover
point as me,
you can just
call
1-800-769-0747
and fire off
your order. If
you have VSR
VR models
(that includes
the 4, the 4
Gen. II, the
4.5, the 6 and
the 8), you'll
be happy to
know that this
is an
Albert-approved
project!

As
the drawing
indicates, we
will need four
of each part,
one set to be
used in each
branch of our
circuit, with
two complete
units
necessary for
stereo. First,
I used 4
InfiniCap
Signature
Series 0.47
micro Farad (μF)
metalized
polypropylene
capacitors. My
pictures show
a different
iteration,
made with
Hovland
MusiCap’s®
instead of the
InfiniCaps,
but they have
since been
replaced with
the slightly
more open and
detailed
InfiniCap
"D"
series after
experimenting
with a variety
of caps. These
are rated at
425 volts at
10% tolerance
and go for
$9.95 for the
D-type (part
number 58937),
and $10.95 for
the S-type
(part number
58954). What’s
the dif? Well,
the
"S"
series is said
to be voiced
with a
"single-ended
tube-type
sound,"
while the
"D"
stands for
Direct. Next I
ordered four
2.2 k-ohm
Holco
metal-film
resistors,
part number
53813. These
are precision
¼ watt
metal-film
jobbies that
sell for $0.40
each and have
a tolerance of
0.05%!

You
will want to
use some form
of insulation
on either the
cap or
resistor leads
if necessary,
especially
when using the
larger
experiment
boxes. You may
choose to
insulate both,
but won’t
likely need to
do more than
just the caps,
because the
assembly of
our circuit
will avoid any
other
proximity
problems. You
can use the
insulation you
strip from the
hook-up wire
to insulate
the bare
leads, but
will of course
have to
increase, once
again, the
amount that
you plan to
use. The
important
thing here is
that you want
to preclude any
possibility
for shorting
within the
boxes.

Gentlemen,
Start Your
Soldering
Irons

First,
drill pilot
holes with the
small (I used
a 3/32")
drill bit in
both 2"
panels and one
of the 3"
panels of the
boxes. Be sure
to use the same
3" side
on both boxes
for symmetry.
Once you’ve
gotten them
drilled,
change to the
¼" bit
and enlarge
the holes.
Make sure to
de-burr the
holes so the
jacks will fit
flush and
snug. Open the
three packages
of jacks, and
using a pair
of pliers,
bend all the
ground tabs on
the ground
connection
rings to 90
degrees for
ease of
installation,
then install
all the red
(right) ones
in one box and
all the white
(left) ones in
the other.
Removal and
reinstallation
of
interconnects
will cause the
jacks to
rotate over
time, thereby
loosening,
shorting, or
breaking the
connections
inside the
box. So be
sure to
install them
snuggly.

Now,
with the first
box oriented
with the jack
in the 3"
face is
pointing away
from you,
install the
first cap
between the
plus (2)
terminal on
that jack and
the plus (4)
terminal on
the left jack.
This is the
high-pass
side. Then,
carefully bend
the leads on
the first
resistor so
that it will
fit between
the plus (4)
terminal on
the left jack
and the ground
ring (3) on
that same jack
(see diagram
and photos).
Carefully
solder the
plus (4) jack
on this side.
DO NOT solder
the second
side of the
resistor to
the ground
ring (3), just
yet—there is
another
connection to
be made to
this post.
Next, insert
the second cap
(because of
size) between
the plus (6)
terminal and
ground ring
(5) on the
right jack.
Then, install
the second
resistor
between the
plus (2)
terminal on
the input
jack, and the
center
terminal (6)
of the right
jack, where we
have already
installed one
end of the
second cap.
Carefully
solder both
this
connection
(6), and the
center
terminal on
the input jack
(2). Finally,
measure and
install the
hook-up wire
from the
ground lug (1)
on the input
jack to each
of the other
two ground
rings, the
left jack
ground ring
(3) with the
capacitor lead
waiting, and
the right jack
ground ring
(5), with the
resistor lead
waiting, and
solder all
three. We now
have one half
of the system
done, with our
high pass (to
feed the
high-frequency
amplifier) to
the right and
our low pass
(to the
low-frequency
amplifier) to
the left—when
the input jack
is facing away
from us. Seat
the box lid
and screw it
down securely.
Repeat the
process for
the second
box.

Using
your Volt/Ohm
Meter, set it
to the first
resistance
setting
available that
is higher than
2.2 k-ohms,
and do some
testing on
your finished
boxes to be
sure you’ve
made no
mistakes. You
should see the
following
readings
across these
contact
points:

If
you get the
above
readings, it
is safe to
assume you
have assembled
the box
correctly. If
any of them
vary, open the
box and double
check all of
your
connections.
Neither The
Stereo Times,
nor I , can be
held
responsible
for any
damages or
problems you
may incur as a
result of
building or
implementing
this
do-it-yourself
project.
Proceed at
YOUR OWN RISK.

Let’s
get
"meddling"!
Insert the
interconnects
from your
preamp’s
outputs into
the input
jacks on both
crossover
boxes, right
to the
red-jack box
and left to
the white-jack
box. Install
one of the new
sets of
interconnects
from the
high-pass
output side of
both boxes to
the respective
right and left
inputs of the
high-frequency
amplifier.
Finally,
install the
last set of
interconnects
from the
low-pass
output side of
both boxes to
the respective
right and left
inputs of the
low-frequency
amplifier.

Now
all you have
to do is match
the volume
levels between
the two amps.
This can be
done by ear,
but I would
recommend
using a
Decibel meter
and a test
disc. You will
want to select
test tones
that are at
least one full
octave above
and below your
band pass
frequency. For
my chosen 120
Hz crossover
point, that
would mean at
least 60 Hz or
lower for
low-pass side
and 240 Hz or
higher for the
high-pass
side. After
doing a
listening test
at a very low
volume to be
sure that you
will not hurt
the amps, and
to verify that
you have done
your bench
work properly,
select a
preamp level
setting that
will
approximate
your normal
listening
volume. The
reason for
this is that
the amp-to-amp
balance will
vary slightly
with the
volume
setting, so
you will want
to optimize it
for your
normal
listening
level.

Set
the dB meter
on a tripod
aimed at your
listen area,
or rest it on
an
unobstructed
location on
your listening
seat (atop the
seat back for
example), and
turn it on.
Now, run the
test tone for
the amplifier
that does
not have
variable
volume
controls and
note the SPL
reading on
your meter.
Once you have
noted that
reference dB
level, run the
test tone for
the other
amplifier and,
without
changing the
volume setting
on the preamp
or moving the
SPL meter,
adjust the
volume
controls on
the second
amplifier
until the dB
level matches
the reference
reading
obtained from
the original
amplifier
test.

In
my system,
besides noting
an obvious
increase in
power and
extension to
the lower
frequencies,
the mids and
highs were
presented with
a greater
level of
clarity and
detail. Bass
runs were
clearer, more
easily
delineated,
and downright
room-shaking.
The
soundstage,
though
exceptionally
good before,
was somewhat
more expansive
and a bit
better
focused. Plus,
I could get a
little more
SOUND PRESSURE
out of the
rig. When the
Genie speaks
in Roger
Waters’ Amused
to Death
(Columbia
468761 0),
doors rattled
in their
frames (guess
I have more
room taming to
do)! When the
lowest organ
note in
Saint-Saёns’
Symphony
No. 3 (RCA
LSC 2340 or
Mercury SR
90012) was
keyed, my room
pressurized
akin to the
way it feels
in the
presence of a
real pipe
organ. Now I
am more able
to sit
comfortably—instead
of preparing
to dive for
the volume
control when
playing my
favorite
dynamic pieces
at near
room-shaking
volumes. Not
too shabby for
less than $50
worth of raw
components,
some
interconnects,
and a second
amp, eh?

Understand
that you may
notice subtle
voicing
changes over
the first 20
or 25 hours of
use—perhaps
even longer.
This is common
and is due to
the final
settling of
the electronic
components and
new cables.
Now, put on
your favorite
music, sit
back, and
enjoy. You’ve
earned it!